Well, the Shrieking Shack is outside the Hogwarts boundaries, so if he died there, the castle would have no way of knowing that he's dead. And it really isn't as if a Headmaster has never left the castle in an emergency. Albus was hardly ever around when the balloon went up and he was needed. Plus, they hadn't yet recovered Snape's body and brought it back to the castle when the story ended.

Not that Rowling cares, but Albus's portrait only showed up (for certain) after his body had been brought *into* the castle.

But it still doesn't excuse the whole determination to ignore the fact that Snape was making a *necessary* strategic retreat.